In an electronic article surveillance (EAS) system of known type, a transmitter-receiver arrangement is disposed aside an area to be controlled and transmits a first, high-frequency, signal into the area. A separate transmitter furnishes a second signal of substantially lower frequency (commonly referred to as the E-field or electrostatic field signal). Reradiators, typically comprising a dipole and a nonlinear element, are responsive to the incidence thereon of both transmitted signals to transmit a composite thereof and detection of such composite signal in receiving apparatus indicates the presence of the reradiator (security tag) in the controlled area. Such system is further described in U.S. Pat. Nos. 3,895,368 and 4,139,844, commonly-assigned herewith and incorporated herein by this reference.
The reradiator is incorporated in parent structure customarily referred to as an EAS tag or marker. There is a continued need to reduce the size of such tags. The fundamental problem in producing a small tag flows from the frequencies used in the above-described EAS system, especially the high frequency, which is in the microwave band, e.g., 915 MHz. Tags are most practical when made near one-half wave long, which is approximately 6.5 inches at 915 MHz. Making a tag shorter than such half-wavelength causes it to become electrically too short. The resulting capacitive reactance, combined with the capacitive source impedance of a diode, makes such designs complicated as impedance matching becomes very critical.
Existing solutions use a combination of narrow and wide foil areas or make use of diode leads to overcome capacitive reactances. These methods require much space and either make the tags long, or when tag elements are folded back to reduce length, will make them wide.
One prior art effort is set forth in U.S. Pat. No. 4,642,640, commonly-assigned herewith and to which incorporating reference is hereby made. In the '640 patent, a narrow serpentine foil section is used for impedance matching. The principle underlying the reradiator element of this patent is that of an non-symmetrical dipole, which is folded back to conserve length.
A second prior art effort is set forth in U.S. Pat. No. 4,736,207, commonly-assigned herewith and to which incorporating reference is hereby made. In the '207 patent, the impedance matching is again achieved by narrow foil sections along the edge of the tag. In this case, the narrow sections also form part of the reradiating RF element of a symmetrical dipole principle. Again, as in the '640 patent, part of the pattern is folded back to conserve space.
From the foregoing, it will be understood that the EAS industry would evidently be aided and abetted by tags which are effective in respect of detectability with reduced lengths and widths.